1. Field of the invention
The present invention relates to a recording ink containing at least water and a coloring agent.
2. Description of the Related Art
As the ink used for an ink jet recording method, inks comprising various kinds of dyes dissolved in water or a solvent composed of water and a water-soluble solvent, and various kinds of additives added thereto as required (such inks are hereinafter referred to as dye inks) have been used predominantly, as disclosed, for example, in JP-A-63-51485, JP-A-63-56575, and JP-A-1-198671. However, when the dye ink is used for printing, it results in problems such as insufficient water-fastness and light-fastness of recorded images on a recording medium.
In view of the above, application of an ink using a pigment such as carbon black or Diazo Yellow as a coloring agent in place of the dye has been proposed to improve the problems of the dye inks with respect to water-fastness and light-fastness of recorded images. For example, see JP-A-57-10660, JP-A-57-10661, U.S. Pat. No. 4,597,794 (JP-A-1-15542), U.S. Pat. Nos. 5,160,370, 5,229,786 (JP-A-2-255875) and JP-A-2-276876. Since the pigment ink adheres to a recording medium such as paper by using a resin or the like, the pigment ink is superior in water-fastness to the dye ink. Further, since the pigment is less reactive to light compared with the dye, the pigment ink is superior in light-fastness to the dye ink.
In an ink jet recording system, recording is conducted by filling an ink in an extremely narrow channel and ejecting ink droplets through a nozzle at the top end of the channel by instantaneously applying a physical force to the channel and depositing the droplets on a recording medium such as paper. Accordingly, a low viscosity ink is required for easy flow of the ink through the narrow channel. In the case of an aqueous ink, water is evaporated in the nozzle, which can cause a coloring agent to remain in and sometimes clog the nozzle, and thereby make recording impossible. A water soluble solvent of low volatility is usually added in an effort to prevent clogging in the nozzle, and an ink having a viscosity of 1 to 3 centipoises (cps) at 25.degree. C. has been used.
When ink jet recording is conducted by using an ink having low viscosity of 1 to 3 cps at 25.degree. C., the energy required for jetting can be reduced because of the low viscosity. However, this in turn results in a problem that splash tends to occur upon jetting of the ink through the nozzle to bring about a deterioration in the print quality. Splash means a phenomenon where a plurality of ink droplets are jetted in various directions from the nozzle.